Temperature regulators
Self-operated temperature regulators belong to the group of direct acting regulators which require no external source for operation. Their characteristic feature is their compact design, including a sensor, a valve and a capillary tube. Their simple operating principle is based on fundamental mechanical, physical and thermodynamic laws. A temperature control loop with a heat exchanger is shown in Fig.1. When the water has left the heat exchanger and circulates in the domestic hot water loop, its temperature must be kept constant. In the heating loop, a heat transfer medium, e.g. hot water, circulates through the heat exchanger and transfers part of its heat to the domestic hot water loop. If we assume that the temperature of the hot water remains constant, the transferred heat quantity depends on the flow rate. The flow of hot water is adjusted by the self-operated regulator. The sensor measures the temperature of the medium to be controlled and converts the measured value into a pressure signal which is used as output variable. The sensor output signal is transmitted via the capillary tube to the operating element where it is converted into a travel which results in a change of the plug position. Temperature regulators obtain their actuating power from the medium to be controlled, so they do not need supply lines or auxiliary devices. This is the most important benefit of self-operated regulators. They keep costs low, while exhibiting high operational reliability.
temperature control loop

self-operated regulator

temperature sensor

heating loop

domestic hot water loop heat exchanger

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Fig. 1: Temperature control loop with heat exchanger

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Self-operated Regulators ⋅ Temperature Regulators

Sensors
Sensors are used to measure the temperature of the medium to be controlled. A good sensor must fulfill two important requirements. It must respond quickly to temperature changes and provide accurate values of variables that
measurement is based on three methods

change over time. The self-operated regulator measures variables according to the three following principles:

4 liquid expansion 4 adsorption 4 vapor pressure
These principles utilize the change in volume, in structure or the conversion of a matters state of aggregation. Liquid expansion principle When measuring the expansion of a liquid, the quality of the results depends to a great extent on two factors: the sensor volume and the specific heat capacity of the filling medium.

V2.0 h 2 V2

1: sensor 2: operating element 3: cylinder
3 1

h2

h2.0

V1=V2 V1.0=V2.0

d2

h 1 < h2 h1.0 < h2.0
h1.0

V1.0 V1
h1

sensor
Fig. 2: Expansion of a liquid in a cylinder

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d1

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 Sensor volume
Solids, gases and almost all liquids expand when the temperature increases. This physical principle of expansion is utilized by thermometers. An increase in temperature causes the liquid level in a capillary to rise. This level is indicated on a scale. A sensor operating on the liquid expansion principle is shown in Fig. 2. The liquid expands in the cylinder when the temperature rises. As the wall of the cylinder prevents lateral expansion, the liquid expands only in the axial direction, pushing the piston and the connected pin upward. The increase in volume can be calculated as follows:
expansion in the cylinder

∆V = V0 γ ∆T

The expansion of the filling medium is determined by two factorsthe specific coefficient of expansion γ which depends on the type of fluid used and the change in temperature ∆T. The height of the pin protruding from the cylinder is a measure for the expansion and represents a function of the temperature h=f(T). To achieve a particular travel of the pin ∆h, the shape of the operating element must be considered and adapted as required. Generally, small sensor volumes yield larger travels than large volumes (Fig. 2). In instrumentation, small working cylinder areas or narrower working cylinders are preferred since the measuring span is better represented when the pin travel is large. In this way, more accurate measurement results are obtained. However, a disadvantage of small-volume sensors is the low power transmission. A movement of the valve, though, always requires an (actuating) force. When sizing a sensor, a compromise must be found between the change in travel and temperature as well as the increase in force.
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absorb and release should be as low as possible. This can be achieved either by keeping the volume or the mass low, or by choosing a filling fluid with a low specific heat capacity. The quantity of heat stored in the fluid calculates as follows:

W = cp m ∆T

cp is the specific heat capacity, m the mass and ∆T the change in temperature in °C. Note that the specific heat capacity is not constant, but changes with the temperature.
water not suitable as filling medium

Due to its high specific heat capacity, water is not suitable as filling medium. It has yet another disadvantage: With the exception of water, all liquids expand continuously with increasing temperatures and condense when the temperatures fall. Water, however, reaches its highest density at 4 °C and expands at higher as well as lower temperatures. Therefore, the temperature measured in these ranges would not be clear. SAMSON temperature sensors use low-viscous, synthetic oil as filling medium. This liquid is harmless, i.e. it endangers neither health nor environment. It can be discharged with the waste water if leakage occurs (water danger class 0). Formerly used silicone oils were not accepted by the automotive industry since silicone oils cause wetting problems with water-based lacquer. Apart from liquids, resins and elastomers can also be used as filling fluid.

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Adsorption principle

CO2

activated carbon

CO2

T 1, p 1

T1 > T 2 p1 > p 2

T 2, p 2

Fig. 3: CO2 molecules depositing on activated carbon

The adsorption principle is based on a physical method. The temperature sensor contains activated carbon and carbon dioxide. When the sensor is heated by the medium to be measured, the activated carbon releases CO2 molecules. The pressure inside the sensor increases (Fig. 3), representing a significant value for each temperature value. When the internal pressure is transmitted via a control line to the operating bellows, the valve position is changed with respect to the temperature. The most important benefit of the adsorption principle is its good adaptation to the respective application. The measuring span of an adsorption sensor can be set in two ways:
flexible application... activated carbon releases CO2 molecules

Vapor pressure principle The vapor pressure principle is based on a thermodynamic method. When a liquid is subjected to heat, it begins to boil at a certain temperature and steam is generated.

propane
100

n-butane

n-pentane
10

n-heptane

pv [bar]
1 0.1 -50

0

50

100

150

200

T [°C]
Fig. 5: Steam pressure curves of hydrocarbons

The boiling temperature, however, depends on the prevailing pressure. The lower the pressure, the lower the temperature at which the liquid starts to boil. Example: In an open vessel, water boils at 100 °C. The boiling temperature in a pressure cooker, however, is considerably higher because the pressure created in the airtight cooker is much higher. The steam pressure curves of hydrocarbons are plotted in Fig. 5. When the
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sensor system utilizes steam pressure curve

temperature of the medium to be measured increases, the boiling pressure in the closed sensor system increases as well, following the rising steam curve.

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Self-operated Regulators ⋅ Temperature Regulators

Depending on the measured temperature, a significant pressure is created in the sensor. The internal sensor pressure acts on a bellows in the thermostat, generating a thrust. The filling medium in sensors for self-operated regulators often is a mixture of hydrocarbon compounds (HC-compounds). The maximum ambient temperature must be minimum 15 K lower than the set point to prevent the filling medium from vaporizing in the control line. The basic properties of the different measurement methods are compared in the following table.

Sensor thrust expansion behavior excess. temp. safety mount. position

liquid expansion strong linear

adsorption weak linear

vapor pressure medium not linear

low any

high any

medium defined

Table 1: Properties of different sensor systems

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How the sensor design influences the dynamic behavior

Y water
1

0.6 0.2

air bulb sensor

Y
1

200

400

600

800

t [s]

water
0.6

four-bulb air sensor

0.2

200

400

600

800

t [s]

Fig. 6: Unit step response of a bulb sensor and a four-bulb sensor

Types of bulb sensors Bulb sensors are in direct contact with the medium. The resulting heat exchange is characterized by the heat transfer coefficient. The heat transfer coefficients of liquids are remarkably higher than those of
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sensors require large heat transfer surfaces

gases. Temperature changes of a liquid act therefore faster on the sensor case, the filling medium and finally the valve position. When sizing the temperature sensor, the surface provided for heat transfer must be as large as possible. While the cylindrical surface of a bulb sensor is sufficient for mea-

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Self-operated Regulators ⋅ Temperature Regulators

suring water and other liquids, gases require a specially manufactured four-bulb sensor. In this sensor, the ratio between the sensor surface and the volume of the filling medium is larger than that of the bulb sensor. Fig. 6 compares the unit step response of a bulb sensor with that of a four-bulb sensor after they have been immersed into warm circulating water and into an air duct. The temperature difference is so big that the pin passes through its entire travel. Particularly in the air duct, the larger sensor volume proves favorable. Compared to the four-bulb sensor, the pin of the bulb sensor requires almost three to four times as much time to reach its final travel. Set point adjustment
control action of self-operated regulators

Self-operated regulators usually exhibit proportional control action (P regulators). In the case of self-operated temperature regulators, the proportional action causes the valve travel h to change proportionally with the measured temperature T. The proportional-action coefficient is Kp (formerly: proportional band Xp; Xp = 100 %/Kp). The following equation describes the control action for temperature regulators.

∆h = Kp · ∆T

large travel at small ∆T

As described in the Control Engineering Fundamentals (see also Lit. [2]), P regulators have a steady-state error. When the steady-state error is to be kept small, a large proportional-action coefficient is required (small proportional band). This means for the temperature regulator that a large travel must be achieved at a small ∆T. The measuring span of the sensor becomes accordingly smaller.

universal application requires set point adjuster

However, narrow measuring spans are an obstacle to the universal application of sensors. Therefore, the temperature regulator is equipped with a set point adjuster.
SAMSON AG ⋅ V74/ HRB

Example: In systems with volume changes, an externally adjustable piston can be moved to change the volume of the system. When the piston is pushed into the right cylinder, the pin in the operating element is lifted, providing the

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required volume. As a result of the changed pin position, the travel position of the valve is changed, too (Fig.7).

pin

piston

operating element

Fig. 7: Example: Set point adjustment at temperature sensor (system based on change in volume)

Excess temperature When the temperature reaches the upper limit of the set point range (closing temperature), the pin is fully extended. The valve is in its end position. When the temperature rises above this value, the liquid in the sensor cannot expand further. If no equalizing volume is provided, the rising internal pressure will damage the sensor. To prevent this, a pressure relief fitting is installed (Fig. 8). When excess temperatures occur, the rising filling pressure acts on the piston bottom and pushes the piston out of the sensor against the force of the excess temperature spring. This increases the sensor volume. Excess temperatures only occur under the influence of externally supplied energy, in case of a defective valve (the valve does not completely close) or with extremely oversiSAMSON AG ⋅ 00/09

sensor protected against excess temperature

zed valves. Decreasing the set point will not help, since the valve is already closed in this state. In the end, decreasing the set point results in a defective device.

A prerequisite for the proper functioning of temperature control systems is the optimum location of the sensor. It should be totally immersed in the medium to be measured (see Fig. 9).

dead times must be avoided

Another important requirement is that the sensor measures nearly without dead time. Dead times occur, for example, in a heating system when the sensor is not located directly at the heat source, e.g. the heat exchanger, but far away in the heating pipe. In this case, temperature changes are measured with delay. These dead times can cause the system to oscillate and can trigger the safety mechanisms due to excess temperatures created. In addition to the aspects to be considered in sensor positioning, the dynamic behavior of the sensor also plays an important role for the heat transfer.

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a) correct

b) correct

c) permissible if it cannot be avoided

d) incorrect

Fig. 9: Sensor locations

Ad a, b) The sensors entire length is fully immersed in the medium Ad c, d) The sensor is only partially immersed in the medium

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Self-operated Regulators ⋅ Temperature Regulators

Dynamic behavior of sensors

x
1

A B C sensor

x 0.63
0.2 20

x
1

τA τB

40

60

80

100 120

τC

t[s]

B
0.6 0.2 20 40 60

C
80 100 120

sensor with thermowell t[s]

τB

τC

Fig. 10: Unit step responses of sensors

The dynamic behavior of a self-operated regulator depends on the dynamic behavior of its sensor. The dynamic behavior is characterized by the time constant τ. The constant describes the time the pin needs to reach approximately 63 percent of the new operating point when forced by a step change in temperature.

for : t = τ ⇒
18

x ( τ) = 0.63 = 63% x max

SAMSON AG ⋅ V74/ HRB

x (t ) −t = 1− e τ x max

(

)

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When looking at the sensor from the viewpoint of control engineering, the sensor can be regarded as energy store. Its dynamic behavior can be described by means of an exponential function using the time constant T1 = τ (first-order delay). When mounting a thermowell (see chapter Accessories), another energy store is added to the system. Hence, a second-order system is created. To describe such a system, the time constant Tu and the build-up time Tg can be used. For further details, please refer to the Technical Information L102 EN. As can be seen in Fig. 10, small time constants are typical to fast-responding sensors. Table 2 lists the time constants of the different SAMSON sensors. Measurements were made in water. You can see from the table below that the use of a thermowell causes long delays.

It practically eliminates the fast response times inherent to sensors, and they are almost as slow as liquid-expansion sensors.
sensor material: bronze and copper

Standard materials for sensors and thermowells are usually copper or bronze because of their excellent conductivity. For corrosive media, stainless steel versions are used. When a stainless steel sensor is used, the time constant increases by approximately ten percent compared to copper sensors. With thermowells, the time constants of copper and stainless steel versions are nearly identical. Thermowells are not suited to be used with sensors for air. Due to the special sensor shape, a narrow air gap is formed between the thermowell and the sensor, which has an insulating effect. The time constant of an air sensor with thermowell would be much higher than that of a standard sensor with thermowell. NOTE: In addition to the time constant τ, variables such as T0.5 (half-value period) or T0.9 (90% value) are used to describe the dynamic behavior of sensors. These values can be calculated for first-order systems using the equation below and the time constant τ:

Valves and their applications
Force-balancing The signal pressure of self-operated regulators is generated by the expansion of the filling medium in the operating element. To make the interaction of the different forces understandable, a valve balanced by a bellows is described in the following example (see also Technical Information L202EN). The upstream pressure p1 and the downstram pressure p2 acting on the valve plug are balanced by the bellows. As a result, the actuating force FA is opposed only by the pre-loaded spring FF (Fig. 11). Both forces are balanced in a state of equilibrium.
spring and actuating force are balanced in a state of equilibrium

The self-operated regulator is used to reduce or increase the flow rate when the temperature at the measuring point rises or falls. The temperature is regulated as follows:

4 When the medium heats up, e.g. due to a reduction of the flow rate, the filling liquid in the operating element expands and exerts the actuating force FA on the valve.

4 The valve closes against the spring force FF, reducing the flow of the heating medium.

4 When the flow is reduced, the temperature falls until a new equilibrium of
forces and, hence, a new valve position is reached. NOTE: When sizing a system including a heat exchanger, the upstream temperature must be minimum 10 K above the set point temperature to ensure safe closing of the valve. Globe valves in cooling service
reversing device changes operating direction

The globe valves described above close when the temperature at the sensor rises, hence, they are suitable for heating service. In cooling installations, however, a system is required that opens the valve with increasing temperature to release the cooling medium.

valve connection to operating element

closing spring

Fig. 12: Reversing device for globe valves in cooling service (valve is closed by the spring force of the reversing device and opens when the temperature rises)

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This is achieved either by installing a reversing device between the 'normal' globe valve and the operating element (Fig. 12), or by changing the seat/plug position (Fig. 13).

Three-way valves Heating and cooling control systems require different valve styles. Globe valves control one flow to adjust the desired temperature. Three-way valves, on the other hand, mix or divert two heat flows. Three-way valves have three ports (A, B, AB), while globe valves have two. When no actuating force is exerted on the valve, a return spring ensures that the plug is firmly placed on one of the two seats.
medium flow through mixing valves

In mixing valves (Fig. 14), the heating medium enters at port B via the seat/plug assembly and leaves through port AB. Port A is closed. When an actuating force acts on the plug stem, the valve moves towards its other end position, reducing the flow through the inlet port B and opening the inlet port A.

B

A

AB

Fig. 14: Three-way mixing valve

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The flow through diverting valves (Fig. 15) is quite different. Here, the process medium enters at port AB. The streams are diverted according to the valve position and finally leave through the ports A and B.

medium flow through diverting valves

B

AB

A

Fig. 15: Three-way diverting valve

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Self-operated Regulators ⋅ Temperature Regulators

The operating principle of the valves and their application in a heating and a cooling system are illustrated in Figs. 16 and 17.

control task: constant temperature in the consumer loop

heating system

flow control

mixing

diverting

globe valve

three-way mixing valve

three-way diverting valve

B AB A A

B AB

when temperature increases A opens B closes A closes B opens

valve closes

Fig. 16: Example of a heating system
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Figs. 16 and 17 (heating/cooling) show typical installation examples where the valves can be installed either in the flow pipe or in the return pipe. In heating systems with high temperatures and low pressures, cavitation can cause problems, therefore the valve should be installed in the cooler return pipe.

installation of valves in heating or cooling systems

control task: constant temperature in the consumer loop

cooling system

flow control

mixing three-way mixing valve

diverting

globe valve

three-way diverting valve

B AB A A

B AB

when temperature increases

valve opens

A opens B closes

A closes B opens

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Fig. 17: Example of a cooling system

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Self-operated Regulators ⋅ Temperature Regulators

When engineering the heating or cooling installation, it must be ensured that the process medium flows in the opening direction of the plug of the mixing or diverting valve so that "vibrations" near the closing position are prevented. The small surface, the high velocity and the low pressure would otherwise cause the plug to be seized in the seat and released again when the flow is interrupted. Safety engineering and combination technology One important field of application for temperature regulators (TR) is in safety engineering. The following safety equipment can be differentiated:

 Safety temperature monitor (STM)
The STM closes the valve in heating operation and opens it in cooling operation when the limit value is exceeded or when the device is defective. Once the temperature lies within the limit value range again, the control function is automatically reactivated.

 Safety temperature limiter (STL)
The STL closes the valve in heating operation, opens it in cooling operation and locks the valve when the limit value is exceeded or when the device is defective. Once the temperature lies within the limit value range again and after mechanical unlocking (loading the closing spring), the control function is automatically reactivated.

 Combination technology
STMs and STLs can, of course, be combined with other TRs. For reasons of safety, systems can also be equipped with a combination of STL and STM.

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Accessories
The following accessories are designed for use with temperature regulators and are available for different applications:

 Thermowells and flanges
Thermowells are used to protect the sensor from the medium. This is important, for instance, when chemically corrosive media would damage the sensor material. Thermowells can be additionally coated with PTFE for protection. Basically, sensor flanges can be designed with and without thermowells. Flanged thermowells are frequently used for high nominal pressure ratings (> PN 40). Using thermowells has another big advantage: The process medium can remain in the system when a sensor must be replaced.

 Distance pieces
Distance pieces are used to seal the valve/thermostat connection. At the same time, they isolate the operating element of the thermostat from the process medium flowing through the valve.

 Double adapter
A double adapter enables the connection of additional controlled variables. In a heating system, for instance, a double adapter can be installed to regulate the return flow temperature limitation in addition to the main task of regulating the desired heating temperature.

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Self-operated Regulators ⋅ Temperature Regulators

 Extension pieces
For applications with heating media exceeding a temperature of 220 °C, extension pieces are used to prevent the heat from being radiated on the operating element. Contrary to distance pieces, extension pieces do not require an internal seal, because they are not primarily used for the isolation of the sensor from the medium. In case special applications should require a seal, this seal can be integrated in the extension piece.

 Electric signal transmitter
Electric signal transmitters can be used to indicate different operating states. Depending on the application, information such as Valve closed or even Safety temperature limiter triggered could be of interest.